Process for producing sponge iron

ABSTRACT

Fine sponge iron particles are mixed with iron ore and the mixture is then sintered before being charged to a kiln for reduction into sponge iron.

United States Patent Wetzel et al. 1 Mar. 7, 1972 [54] PROCESS FORPRODUCING SPONGE {56] References Cited IRON UNITED STATES PATENTS [72]Invemm m'f gfi mf hfi jfg mg 1,864,593 6/1932 Gustafsson ..75/34 W2,826,487 3/1958 Davis, Jr. ...75/33 X [73] Assignee: Fried KruppGesellschaft mit beschrankter 3,428,445 2/1969 Rausch et al. ..75/3Haftung, Essen, Germany 3,489,549 1/1970 Jomoto et a]. ..75/5 Filed: p1969 3,497,348 2/1970 Rausch et al. ..75/33 [2]] App]. N 855,278 PrimaryExaminer-L. Dewayne Rutledge Assistant Examiner-J. Davis AuomeySpencer &Kaye [30] Foreign Application Priority Data Sept. 7, 1968 Germany ..P 1758 951.4 [571 ABSTRACT Fine sponge iron particles are mixed with ironore and the U.S. mixture is then sintered before being charged to a for[51] Int. Cl. ..C2lb 13/14 reduction into Sponge iron [58] Field ofSearch ..75/3, 5, 33, 35

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KILN smuaE IRON AND MR NIZED .BROWN 0A1 v Inventors: Pol-[- EmLL Debi 2L\OLLLL D'anssrm Hk-kornass PROCESS FOR PRODUCING SPONGE IRON BACKGROUNDOF THE INVENTION The present invention relates to a process forproducing sponge iron using sintered ore as the material charged to thereducing apparatus.

The production of sponge iron includes the direct reduction of iron oreswhich have been mixed with reducing agent. In the case of orescontaining fine particles, it is often required that the fine particlesbe brought together into lumps, before charging the reducing apparatus,in order to obtain sponge iron in lump form.

There are a number of processes for the production of sponge iron. Inone of these processes green pellets are hardened on a traveling grateat low temperatures and then reduced to sponge iron in a rotary kiln.

Besides pelletizing, there is the sintering method for bringing theparticles of a fine ore into lumps. Both of these processes are suitedfor use in the production of sponge iron.

Which process is actually used is primarily a question of economics.

In the case of pelletizing, the fine ore must be ground in ball or rodmills and then formed into pellets. The pellets must then be burned.

Sintering is effected by a burning of solid fuel previously mixed withthe fine ore. The energy costs are essentially determined by the amountof solid fuel mixed.

SUMMARY OF THE INVENTION An object of the invention is to reduce thecost of the sintering method of lumping fine iron ores for the chargingof reducing apparatus for forming sponge iron and to consequently makethe sintering method a better alternative for the pelletizing method,especially where the cost of grinding and forming fine ore into pellets,followed by burning, is especially high.

This as well as other objects which will become apparent in thediscussion that follows are achieved, according to the presentinvention, by at least partially replacing the prior art solid fuel inthe sinter mixture by sponge iron. In an especially economic form of theinvention, the undesired fines from a sponge iron producing, reducingapparatus are fed back and mixed with the fine ore before sintering.Such sponge iron fines have provided difiiculty for the furthertreatment of sponge iron coming from a reducing apparatus and it hasbeen necessary to agglomerate them in order to be able to feed them onwith the larger sponge iron particles. The strength properties of thesintered material obtained using sponge iron of minus 3 millimeterparticle size is not appreciably altered from that obtained using mixescontaining only the solid carbonaceous fuel of the prior art.

It has been found in practice that the greatest economy is achieved ifthe particle size of the sponge iron fines used for the sinter mixtureis held below 3 millimeters, since sponge iron above that size is morevaluable as an iron product than as an additive for a sinter mix.

From a knowledge of the amount of sponge iron fines available forrecirculating into the sinter mixture and a knowledge of the amount ofsolid carbonaceous fuel which previously had to be put in the sintermixture, it is possible to determine how much of the solid carbonaceousfuel can be replaced by the sponge iron fines. The amount x of solidfuel that can be replaced is given by the following formula:

FY rs/ e (l where y equals the weight of sponge iron fines in kilograms,H equals the amount of heat liberated per kilogram of sponge iron fines,and H equals the amount of heat liberated per kilogram of solidcarbonaceous fuel. In determining the H- values, it is assumed that thecarbon is burned to CO and the sponge iron to Fe,O

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow diagram of theinvention. FIG. 2 is a modification of a part of the flow diagram ofFIG.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A sinter mix is prepared fromiron ore, sponge iron, and some water to provide some green strength tothe mix and to maintain porosity in the sinter bed. To this mix may beadded auxiliary components, sinter fines separated from the mate rialcharged to a reducing apparatus, and a solid fuel such as coke fines orcoal dust, should the amount of sponge iron be insufficient to providethe total heat required for the sintering.

Besides its primary meaning of raw ores extracted from the earth, "ironore is meant to include materials such as the iron oxides resulting fromthe oxygen torch cutting of steels, etc.

It has been found that relationships dependent on particle size in thesinter mix are substantially the same for the mix of the presentinvention containing sponge iron as for the mix of the prior artcontaining only solid carbonaceous fuels.

A preferred amount of sponge iron for the sinter mix has been determinedto be from 5 to 25 weight percent. The lower limit results in a completeutilization of the minimum amount of minus 3 millimeter sponge iron thatcan be found to occur in a process such as that of Example I below. Theupper limit is the maximum amount of minus 3 millimeter sponge iron thatcan be required in a process such as that of Example II, where no solidcarbonaceous fuel is used.

The sinter mix is loaded onto a moving conveyor and sintered in the samemanner practiced in the prior art for sinter mixes containing only solidcarbonaceous fuel.

The resulting sinter cake is crushed in a roll crusher and this crushedproduct is fed while still hot to a size-separating device such as agrizzly, whereupon the undersize is fed back to the mixer for the sintermix. The oversize is mixed with solid fuel and desulfurizing materialand then put in a rotary kiln for reduction. Examples of suchdesulfurizing material are raw dolomite and limestone.

The sponge iron issuing from the rotary kiln is subjected to a sizeseparation operation by screening to remove the fraction under 3millimeters. This minus fraction is fed back to the sinter mixer. Theoversize sponge iron is treated further as desired, for example in anarc furnace.

As the solid fuel for mixing with the charge to the rotary kiln in thefinal reducing operation to change the sintered material into spongeiron, it is preferred to use brown coal or similar solid fuels havingexcessive amounts of volatile components. In the claims and drawings,brown coal is meant to include equivalents of brown coal with regard tovolatile component content. Any carbonized brown coal remainingfollowing the reduction can be fed right along with the minus 3millimeter sponge iron back to the sinter mixer, if the rate of minus 3millimeter sponge iron production should be insufficient to provide therequired amount of heat for the sintering operation. On the other hand,the undersize mixture of sponge iron and carbonized brown coal can beseparated by a magnetic separator operation. In a preferredmodification, the undersized fraction is not separated in a magneticseparating operation, while the oversize is separated in such manner,the oversize carbonized brown coal being mixed into the kiln charge andthe oversize sponge iron being fed on for further processing.

A preferred flow diagram of the process of the invention is shown inFIG. 1. In addition to the process operations discussed above, FIG. 1shows a tube drier operated by the hot gas exhaust of the kiln. Thebrown coal is dried and preheated in this tube drier before being fed tothe mixer to be mixed with the oversize sintered iron ore to form thekiln charge. Use of the tube drier improves the heat economy of theentire process.

FIG. 2 shows a modification of the flow diagram of FIG. I. Only asufficient amount of the unchanged part of FIG. 1 has been redrawn toindicate the setting of the modification. In this modification, alow-temperature coking apparatus is used to first produce carbonizedbrown coal from the raw brown coal. The coking apparatus is driven bythe hot exhaust gases of the kiln. The gases given off by the raw browncoal are used to heat the kiln and to operate the sinter bed igniter.The carbonized brown coal is fed while still hot to the kiln chargemixer. This modification allows better utilization of the availableenergy in the brown coal.

Representative of the practice of the present invention are thefollowing examples EXAMPLE I A sinter mix was prepared with 20 percentsinter fines, 8 percent water, 3 percent coke dust, 10 percent spongeiron fines, remainder Kiruna ore. This mix was charged on a conveyor asshown in FIG. 1 and ignited to produce sintering. The sinter cake wassubjected to the Micum drum test, which gave the minus 10 millimeterparticle size as 23.5 percent. Thus, a sufficient sinter strength isobtained. Reduction of this sintered material in a rotary kiln gave asponge iron having a total iron content of 75.5 percent and azero-valent iron content of 69.3 percent. Dividing 69.3 by 75.5, it isseen that a metallizing efficiency of 92 percent is achieved. The spongeiron minus 3 millimeter fraction amounted to 7.8 percent, while theamount below millimeters was 14.8 percent.

Kiruna ore is a magnetic ore, whose essential mineralogical component ismagnetite, Fe O The chemical analysis of Kiruna ore is given in Table 1.its ignition loss is 1.0 percent. The sieve analyses for the coke dust,sponge iron fines, and Kiruna ore are given in Table 2. The sinter fineswere minus 3 millimeter material. The coke dust had 87 percent fixedcarbon.

TABLE 1 Kiruna Ore Component Fe 59.2 Fat) 23.8 Fe,0 58.3 Mn 942 P 1.9

S 0.03 SiO, 2.9 Al,0, 0.8 CaO 6.8 MgO L35 TABLE 2 Sieve Analyses of theRaw Materials Raw Material in A Micum drum according to DIN 5 l,7 l 2(DlN=German industrial Standard) was used for the Micum drum test. Ithad an internal diameter of one meter and an effective length of onehalfmeter. The drum was driven at 25 r.p.m. On the inner wall of thecylinder are mounted four baffles at the positions 0, 90, 180, and 270.These bames run parallel to the drum axis and protrude 100 millimetersinwardly toward the axis. Twenty-five kilograms of sinter coke ofparticle size between and 40 millimeters were loaded into the drum. Thedrum was then rotated for 1 minute. The drum contents were then sievedin a stack of sieves to determine the percentage having a particle sizebelow 10 millimeters. According to A. Send and B. Weilandt in themagazine Stahl und Eisen, V. 81 (1961), pages 303 to 310, the amount ofminus 10 millimeter product can be viewed as a measure of the strengthof the sintering. The more minus 10 millimeter material produced, thelower the sinter strength.

The kiln for the reduction measured 14 meters in length and had aninternal diameter of 1.2 meters. The sieve analysis of the crushedsinter cake charged to this kiln is given in Table 3.

TABLE 3 Sieve Analysis of Sinter Product Particle Size in mm. 17

plus 30 [4.8 20-30 28.3

l5-20 18.4 l0-l5 20.4

Total: 100.0

This sinter product was fed to the kiln at the rate of 400 kilograms perhour. The requisite brown coal, lower heating value H =4600 kilocaloriesper kilogram sieve analysis in Table 2, was charged with the sinterproduct into the kiln at the rate of 200 kilograms per hour. More browncoal was blown burning into the kiln by means of an injector at the rateof kilograms per hour, at the sponge iron output side of the kiln. Noother fuel was needed, the total brown coal serving both as reducingagent and as fuel. The temperature at the sponge iron output side of thekiln was l,l00 C., while the exhaust gases left the kiln at the sinterproduct charging side at 870 C.

EXAMPLE ll A sinter mix was prepared with 20 percent sinter fines, 8water, 20 percent sponge iron fines, remainder Kirma ore. This examplerepresents a complete replacement of carbonaceous fuel by sponge iron.This mix was charged on a conveyor as shown in FIG. 1 and ignited toproduce sintering. The minus 10 millimeter fraction in the Micum drumtest was 27 percent. Reduction gave a sponge iron with a total ironcontent of 77.2 percent and a zero-valent iron content of 7 l .9 percentcorresponding to a metallizing efficiency of 93.1 percent. The spongeiron fraction below 3 millimeters amounted to 8.3 percent and that below5 millimeters was l6.l percent. The process variables were otherwisethose in Example I.

EXAMPLE [I] In Example I, the product at the sponge iron output side ofthe kiln had 45 percent carbonized brown coal in the minus 3 millimeterfraction. The analysis of the brown coal as charged into the kiln was asgiven in Table 4. The minus 3 millimeter mixture of iron and carbonizedbrown coal was used to replace the sponge iron fines of the sinter mixof Example I.

The heat evolution was kept the same using formula (1) above.

EXAMPLE rv The 3 percent coke dust of Example I was replaced by a 5percent amount of carbonized brown coal with 52 percent fixed carbon.The sponge iron product from the kiln was first screened as in ExampleIII to remove material of below 3 millimeter particle size. The productof over 3 millimeter particle size was then magnetically separated andthe carbonized brown coal of over 3 millimeter particle size was usedfor mixing with the sinter product in replacement of part of the browncoal used for mixing with the sinter product in Example I when chargingthe kiln. The carbonized brown coal in the plus 3 mm. fraction amountedto 8 percent.

EXAMPLE V EXAMPLE VI In the Example I, a low temperature cokingapparatus in the form of a circulating gas retort made by the firm Lurgiis used for the drying and coking of brown coal in two stages. The hotexhaust gases of the kiln are fed into the coking stage at a temperatureof about 800 C. The resulting coking exhaust gases, which have a heatingvalue of 1,500 kilocalories per standard cubic meter, are partially usedfor the drying stage of the retort. Another part is used as fuel inreplacement of the injected coal for the kiln at the rate of 400standard cubic meters per hour to maintain a working temperature ofl,l50 C. Another part is used for igniting the sinter mix on theconveyor system. Operation at the kiln differs additionally in that the200 kilograms per hour reduction coal is replaced by 250 kilograms perhour carbonized brown coal from the retort. This carbonized brown coalhas a sieve analysis as given in Table 5. The temperature and pressurefor the above mentioned standard cubic meter of gas are Celsius and 760mm.

TABLE Sieve Analysis of Carbonized Brown Coal Particle Size in ntrnv X:greater than 8 B Total:

iron, sinterin the sinter mix bglutilizing for at least part of the heatrequire for srntermg the eat lrberated by burning of the sponge iron insaid mix, changing the product of the step of sintering into spongeiron, separating the minus 3 millimeter size fraction from the spongeiron product of the step of changing, and using said size fraction inthe step of preparing.

2. A process as claimed in claim 1, the step of changing including thestep of mixing brown coal with the product of the step of sintering, thesteps of separating and using incorporating the resulting carbonizedbrown coal minus 3 millimeter size fraction.

3. A process as claimed in claim 2, further comprising the steps ofmagnetically dividing the resulting carbonized brown coal and the spongeiron in the size fraction above 3 millimeters and using the carbonizedbrown coal in the step of changing.

4. A process as claimed in claim 1, the step of changing including thestep of mixing brown coal with the product of the step of sintering, thestep of changing including a reduction step in a rotary kiln, theprocess further comprising the step of preheating and drying the browncoal, before mixing it with the product of the step of sintering, usingthe hot gas exhaust of the rotary kiln.

5. A process as claimed in claim 1, the step of changing including areduction step in a rotary kiln, the process further comprising thesteps of subjecting brown coal to a low temperature coking using the hotgas exhaust of the rotary kiln, mixing the carbonized brown coal in thestep of changing with the product of the step of sintering, using partof the coking exhaust gases from the brown coal for igniting the sintermix in the step of sintering, burning part of the coking exhaust gasesfrom the brown coal, and introducing the hot gaseous products of thestep of burning into said kiln.

2. A process as claimed in claim 1, the step of changing including the step of mixing brown coal with the product of the step of sintering, the steps of separating and using incorporating the resulting carbonized brown coal minus 3 millimeter size fraction.
 3. A process as claimed in claim 2, further comprising the steps of magnetically dividing the resulting carbonized brown coal and the sponge iron in the size fraction above 3 millimeters and using the carbonized brown coal in the step of changing.
 4. A process as claimed in claim 1, the step of changing including the step of mixing brown coal with the product of the step of sintering, the step of changing including a reduction step in a rotary kiln, the process further comprising the step of preheating and drying the brown coal, before mixing it with the product of the step of sintering, using the hot gas exhaust of the rotary kiln.
 5. A process as claimed in claim 1, the step of changing including a reduction step in a rotary kiln, the process further comprising the steps of subjecting brown coal to a low temperature coking using the hot gas exhaust of the rotary kiln, mixing the carbonized brown coal in the step of changing with the product of the step of sintering, using part of the coking exhaust gases from the brown coal for igniting the sinter mix in the step of sintering, burning part of the coking exhaust gases from the brown coal, and introducing the hot gaseous products of the step of burning into said kiln. 